A suspension system for a vehicle preferably provides for changes in toe-angle when wheels of the vehicle receive force from the road surface. For example, when the vehicle is leaning to one side such that the pressure received from the road surface on opposite wheels (i.e., left and right front wheels or left and right rear wheels) is different, the suspension system preferably effects changes in the toe-angle.
Toe-angle refers to the slanting of the front part of the wheel when viewed from above in relation to the direction in which the vehicle is traveling. When the front of the wheel is slanted inwardly, this is referred to as toe-in, while if the wheel is slanted outwardly, this is referred to as toe-out. To improve steering stability during operations such as when turning, it is preferable that suitable toe-in is effected for the rear wheels.
For example, when the driver operates the steering wheel for a right turn, the front wheels pivot by a corresponding amount. Accordingly, the rear wheels trail in the direction that the vehicle is traveling. If the rear wheels spread outwardly (that is, if the rear wheels experience toe-out), a curvature radius formed by a trace of the front wheels and the rear wheels becomes too small such that the vehicle turns by an amount greater than that desired by the driver. This is referred to as over-steer. If such a situation develops, steering control becomes too sensitive such that turning stability deteriorates. That is, it is possible that the driver may have to compensate for over-steer by turning the steering wheel in the opposite direction.
Therefore, it is preferable to effect a suitable amount of toe-in for the rear wheels to realize turning stability. If toe-in of the rear wheels is appropriately provided, a greater curvature radius results such that it becomes unnecessary for the driver to turn the steering wheel in the opposite direction to compensate for over-steer. Steering stability is therefore ensured.
A prior art system to effect toe-in of the rear wheels during braking and turning will now be described with reference to FIG. 1, which shows a plane view of a right rear wheel and a conventional rear suspension system for the same wheel.
As shown in the drawing, an extreme rear end of a lower arm 3 is connected to the vehicle body through bushings and an elastic plate 1. If a braking force is generated, the braking force acts on the wheel 2 along direction X such that a rearward force acts on the entire suspension system. As a result, a front end portion of the lower arm 3 moves diagonally in direction A1 following a bushing connection angle to experience displacement in a downward direction of the drawing (i.e., inwardly toward the vehicle body) by an amount (d).
Also, the rear end of the lower arm 3 moves in the rearward direction (i.e., along direction A2) as the elastic plate 1 undergoes deformation. Therefore, the lower arm 3 rotates counterclockwise (in the drawing) such that a long axis direction of the wheel changes into direction X′ to thereby realize toe-in.
In the case a lateral force is applied to the wheel, only the elastic plate 1 supports the lateral force. However, the elastic plate 1 often provides sufficient strength to fully support such a force such that turning stability is reduced. Vibrations of the elastic plate 1 when traveling on an uneven road surface are also a problem.